PET/CT - Site Planning and Shielding
Design
Melissa C. Martin, M.S., FACR
National Symposium on Fusion Imaging and Multimodalities:
Technical and Regulatory Considerations
Kansas City, KS
February 18-20, 2004
Acknowledgements:
n
AAPM Task Group on PET Shielding – Mark Madsen, Ph.D.,
Chair, University of Iowa Medical Center
n
Jon Anderson, Ph.D. – UT Southwestern Medical Center,
Dallas
n
Judy Reavis, Ph.D. – Hermes Systems Management
Major Considerations for Site Planning
n
Space
n
Power
n
Floor Loading Concerns
n
Radiation Shielding
Factors Affecting Radiation Protection
n
Number of Patients Imaged
n
Amount of Isotope Administered per Patient
n
Length of Time each Patient Remains in the Facility
n
Location of the Facility
n
General Environs of the Facility
Typical PET Facility
Room Layout
Controlled Corridor
Office
Office
Office
Electrical
Utility
PET
Tomograph
3m
PET
Control
Room
Office
Supplies
Bathroom
10 m
PET
Uptake
Room
Office
Cardiac
Stress
Room
Office
Work
Area
Gamma
Camera
Office
Office
Office
Gamma
Camera
Bathroom
Uptake
Probe
Office
Office
Gamma
Camera
Gamma
Camera
Office
PET Exposure Factors: F-18
n
Half Life: 110 minutes
n
Major Radiation Emission: 511 KeV Gammas
n
Half-Value Layer: Several publications list 4.1 mm Pb and
3.4 cm for normal concrete for narrow beam conditions.
Use of these values will not provide sufficient shielding
since they neglect scatter buildup factors.
Tables of broad beam transmission for lead, concrete
and steel based on Monte Carlo calculations performed by
Doug Simpkin, Ph.D., are included in the AAPM task group
report on PET Shielding (currently in review process)
Transmission Data for 511 keV photons
[Douglas J. Simpkin, Ph.D.]
n
Curve fit to Archer Equation
n
Calculated Constants
Transmission
in Lead
n
Transmission
is lower than
constant TVL
Transmission
in Concrete
n
Transmission
is lower than
constant TVL
Transmission
in Steel
n
Transmission
is lower than
constant TVL
Other PET Isotope Data
n
N-13: Half life = 10 minutes
n
O-15: Half life = 2.07 minutes
n
C-11: Half life = 20.4 minutes
n
Rb-82: Half life = 1.3 minutes
n
Ga-68: Half life = 68.3 minutes
n
These isotopes are dominated by the F-18 requirements
F-18 FDG PET Studies
n
F-18 FDG is a non-specific tracer for metabolic activity that
is taken up normally in the brain, heart, bone marrow,
bowel, kidneys and activated muscles, and concentrates in
many metabolically active tumors.
n
To reduce uptake in skeletal muscles, patients are kep in a
quiet state after the administration of the F-18 FDG in either
a bed or chair for 30-90 minutes depending on the type of
scan and the practices of the institution.
n
The patient preparation room is a requirement for any PET
facility and must be included in the radiation safety
planning.
F-18 FDG PET Studies
n
A busy PET facility will often have more than 1 patient in
the uptake room. This must be considered when
performing shielding calculations.
n
Ideally, the busy PET facility will have more than 1 uptake
room.
n
After the uptake period, the patient should void to clear the
radioactivity that has accumulated in the bladder which is
approximately 15% of the administered activity
n
It is highly recommended that a bathroom be reserved for
PET patients
PET Prep Rooms/Toilet
F-18 FDG PET Studies
n
The patient is then positioned on the tomograph for the
procedure and remains in the PET imaging room for 30-60
minutes
n
Patients may be released immediately following the
procedure or may go to a waiting area while the PET study
is reviewed. If the patients are staying in the clinic for any
significant length of time, radiation safety planning must
be performed
n
All areas in the vicinity of the PET imaging clinic must be
considered for shielding calculations including the areas
above and below the PET clinic as well as the adjacent
areas on the same floor
PET Exposure Factors: F-18 FDG Studies
n
For the AAPM Task Group Report, the following assumptions were
made:
n
Calculated Gamma Constant: 0.57 R m 2/mCi hr
n
Dose Rate Constant:
0.55 rem m 2/mCi hr
or when expressed as SI units: 0.147 mSv/mBq hr at 1 meter
(These values are nearly 20% less than the values of Unger and
Trubey listed published tables)
n
Workload for Scanner Room: 8-16 pts/day x 5 days/wk = 40-80 pts/wk
n
Isotope Workload: Assume Patient is injected with 15 mCi (555 MBq)
with an uptake time of 60 minutes
n
Work week = 40 hrs/week
Radioactivity Administration-Dose Factors
n
Patient is the primary source of radiation that needs to be
considered
n
Dose Rate Constant Used in Calcs:
For F-18: 0.188 uGy m2/MBq hr resulting in a dose rate
associated with 37 MBq (1 mCi) of F-18 is 6.96 uGy/hour at
1 meter from an unshielded point source
n
Since the body absorbs some of the annihilation radiation,
the dose rate from the patient is reduced by a factor of 2 or
more. The mean maximum value reported at 1 meter from
the patient just after administration is 3.0 uGy/hour/37MBq
Radioactivity Administration-Dose Factors
n
Radioactive Decay: Because PET tracers have short half
lives, the dose absorbed per hour is less than the product
of the dose rate and the time. The total radiation dose
received over a time period T,D(T), is less than the product
of the dose rate and time by a factor of:
RT = D(T)/(DR x T) = 1.443 x (T 1/2/T) x (1-exp(-0.693 T/T1/2))
For F-18: this corresponds to factors of
30 minutes = 0.91
60 minutes = 0.85
90 minutes = 0.76
Uptake Room Calculations
n
Total dose at a point d meters from the patient during the
uptake time (TU) is
3.0 uSv/hour/37MBq x A0(MBq) x TU (hours) x RTU/d2
If Nw patients are scanned per week, the total weekly dose is
3.0 uSv/hour/37MBq x A0(MBq) x TU(hrs) x RTU x Nw/d2
Uptake Room Calculations
For an uncontrolled area the NRC limit is 1 mSv/year
corresponding to a weekly limit of 20uSv (2 mrem).
Therefore the barrier factor required is:
20 uSv/3.0 uSv/hour/37MBq x A0(MBq) x TU(hrs) x RT x Nw/d2
= 247 d2/(TU(hrs) x RTU x NW x A0(MBq))
= 6.7 d2/(TU(hrs) x RTU x NW x A0(mCi))
Uptake Room Calculations - Example
n
What is the barrier factor required at a point 4 meters from
the patient bed in an uptake room? Pt is administered 555
MBq (15 mCi) of F-18 FDG and there are 40 pts /week.
Uptake time = 1 hour
Barrier Factor = 247 (4)2/(1 x 0.85 x 40 x 555) = 0.21 = 2.45 HVL
Using NCRP values, that requires 1.2 cm of lead or 13 cm of
concrete
Using the Simpkin values, that requires 1.2 cm lead or 15 cm
of concrete
Imaging Room Calculations
n
Most conservative approach is taken where no shielding
from the tomograph is assumed. Method is then similar to
that of the uptake area calculation. Because of the delay
between the administration of the isotope and the actual
imaging, the activity in the patient is decreased by
FU = exp(-0.693 x TU(min) /110) where TU is the uptake time
In most cases, the patient will void prior to imaging removing
approximately 15% of the administered activity
Imaging Room Calculations
n
Barrier Factor =
3.0 uSv/37 MBq x A0 (MBq) x FU x TI(hrs) x RTI x Nw/d2
= 247 d2/ (NW TI(hrs) x RTI x A0(MBq) x FU
For F18 at one hour, the decay factor FU = exp(-0.693x 60/110)
= 0.68
The PET tomograph can provide a substantial reduction of
the dose rate at some of the walls. This depends on the
actual geometry and placement of the tomograph in the
room.
Imaging Room Calculations – Example Calc
n
What is the weekly dose equivalent to a point 4 meters
from the patient during PET imaging. Patients are
administered 555 MBq of F-18 FDG and there are 40
patients per week. The uptake time is 60 minutes ant the
imaging time is 1 hour.
Weekly dose equivalent = 3.0 uSv/37 MBq x 555 (MBq)
exp(-0.693x60/110) x 40 x 1 x 0.85/42 = 65 uSv
What is the barrier factor for occupancy of 1?
247 x 42 / (40 x 555 x 0.68 x 1 x 0.85) = 0.31 = 1.9 HVL
n
Using NCRP Values, this requires 0.95 cm of lead or 10 cm
of concrete. Simpkin values require 0.95 cm of lead or 12
cm of concrete
Calculation for Rooms above and below the
PET Facility
n
Because the 511 keV annihilation radiation is so
penetrating, it is necessary to consider uncontrolled areas
above and below the PET facility as well as those adjacent
on the same level. The following figure shows generally
accepted source and target distances that apply in these
cases.
n
Typically, one assumes that the patient (source of the
activity) is 1 meter above the floor. The dose rate is
calculated at 0.5 meters above the floor for rooms above
the source and at 1.7 meters above the floor for rooms
below the source.
Distances to Be Used in Shielding
Calculations
0.5 m
PET uptake
or scanning
room
0.3 m
Floor to ceiling
(typically 4.3 m)
1m
1.7 m
Distances to be used in shielding calculations
Calculation for Rooms above and below the
PET Facility
n
It should be kept in mind that it may be necessary to shield
the vertical barriers all the way from the floor to the barrier
above (instead of the 7 feet height usually required for xray installations).
n
The vertical barrier need not have the same thickness over
its entire height since the rays from the patient will traverse
the higher portions of the vertical barrier obliquely and
therefore pass through a greater thickness of shielding
material.
Calculation for Room Above an Uptake
Room - Example
n
What is the weekly dose equivalent to a room above an
uptake room? Patients are administered 555 MBq (15 mCi)
of F-18 FDG, the uptake time is 1 hour and there are 40
patients per week. The floor to floor distance is 4.3 meters
and there is 10 cm of concrete between floors.
D = (4.3 – 1) + 0.5 = 3.8 meters
Shielding factor for 10 cm of concrete is 2.5
(3 uSv/37 MBq x 555 MBq x 40 x 1/3.82)/2.5 = 48.3 uSv/week
20 uSv/48.3 uSv = 0.41 = 1.3 HVL
Using NCRP values, this requires 0.65 cm of lead or 6.8 cm of
concrete. Using Simpkin values requires 0.65 cm of lead or
9.3 cm concrete.
Dose Levels in Controlled Areas
n
Dose levels in controlled areas are subject to ALARA
considerations with the maximum limits set to 50 mSv per
year.
n
The technical staff that works directly with the PET patients
receive the largest doses. Components of this dose
include:
Exposure from Patient Injections
Exposure from Patient Positioning
Exposure during Imaging
Adjacent Rooms on the Same Level
n
Annual exposure to occupationally exposed individuals
working in adjoining rooms (without shielding) is expected
to be less than 5 mSv even for a busy PET tomograph.
n
Using the values calculated in the earlier example, the
weekly exposure rate at 4 meters from an uptake rooms
that handles 40 patients per week with an average
administered dose of 555 MBq (15 mCi) is 96 uSv or 5 mSv
per year. This is the ALARA target of 5 mSv/year used by
many clinics.
n
It may be necessary to shield to a lower annual exposure
level in some areas to compensate for the unavoidable
exposure when personnel are close to patients.
Design Considerations
n
Additional shielding is recommended for the Nursing
Stations and the Tomograph Control Room
n
Uncontrolled areas with high occupancy should be located
as far from the PET uptake and imaging rooms as possible
n
If uncontrolled areas are located above or below the PET
uptake and tomograph rooms, the spacing between floors
may need to be greater than normal or additional shielding
added. The floors need to be able to suport the additional
weight associated with additional shielding.
n
Portable lead shields can be used effectively to shield
patients in uptake rooms.
Design Considerations
n
Floors and ceilings typically have 10 cm of concrete which
amounts to about two half value layers
n
Portable lead shields can be used effectively to shield
patients in uptake rooms
PET/CT Room Design
PET/CT Installations
n
Shielding for the CT portion of the PET/CT systems is
substantially the same as for any CT installation
n
If the PET/CT is used only in conjunction with PET imaging,
the CT workload will be considerably less than that for a
dedicated CT system
n
Because the HVL for the CT techniques used is so much
less than that for 511 keV photons, a room conservatively
shielded for PET is unlikely to need additional shielding for
the CT component
PET Shielding Calculations
Barrier
Distance
'D'
Unshielded
Exposure at 'D'
Occupancy
Unshielded
Maximum
Required
Factor
Exposure at
'D'
Permissible
Shielding
(feet)
(mR/hr)
(T)
(mR/wk)
mR/wk
mm
lead
in.
lead
Control Room
12.0
0.43
1.00
17.13
10
2.80
0.11
Control Window
12.0
0.43
1.00
17.13
10
2.80
0.11
Toilet
9.0
0.76
0.25
7.61
2
6.95
0.27
Equipment
9.0
0.76
0.25
7.61
2
6.95
0.27
Corridor 341
8.5
0.85
0.25
8.54
2
7.54
0.30
Corridor 369
10.0
0.62
0.25
6.17
2
5.85
0.23
Door to Corridor
18.0
0.62
0.25
6.20
2
5.90
0.25
Corridor
6.0
1.71
0.25
17.13
2
11.16
0.44
Door to Corridor
9.0
0.76
0.25
7.61
2
6.95
0.27
Hot Lab
4.0
3.86
1.00
154.20
10
14.21
0.56
PET Toilet Walls
8.0
0.96
0.25
9.64
2
8.17
0.32
Prep Room 353
Typical Shielding Requirements
n
Walls will require 0.5 to 1.0 inch lead
n
Doors will need 0.25 to 0.5 inches lead. For door with lead
> 0.25 inches, mechanical assist openers will be needed.
n
Viewing window must be leaded glass or acrylic to match
surrounding walls
Shielding of the PET Tomograph from
Ambient Radiation
n
The PET tomograph itself can be highly sensitive to
ambient radiation, especially in the 3D mode
n
Adjacent patient uptake rooms and other scan rooms must
be considered.
n
One PET vendor specifies a limit of 0.1 mR/hr for its unit
n
This peak exposure rate shielding requirement may require
more shielding than that needed just for personnel
protection where the cumulative exposures are the
concern
Conclusions
n
PET requirements overwhelm any requirements of CT
shielding in combination units
n
Exposures to staff will be close to maximum permissible
limits.
n
Close monitoring of staff exposures will be needed.
n
Shielding will be typically at least 0.5 inches of lead in the
walls with 0.25 to 0.5 inches in the door.
n
Shielding will probably be needed on the ceilings or floors
for facilities with occupied areas above or below these
rooms
AAPM Position Statement (2/2004)
Radiation Protection & Quality Assurance for
PET and PET/CT Systems
In order to assist our members and other interested parties
the AAPM suggests the following guidelines for radiation
protection of PET or PET/CT facilities.
Radiation protection design should be done by a Qualified
Expert familiar with both the clinical operation of a PET
facility and radiation protection design methods for CT
scanners. A detailed Task Group report is being prepared
by the AAPM, and these are meant as interim, general
guidelines.
AAPM Position Statement (2/2004)
General Considerations: The patient is typically injected with
10-20 mCi of F-18 FDG, followed by a resting phase of 3060 minutes and voiding of the bladder prior to entering the
scanner room. Protection of personnel and the public
during this uptake phase should be considered.
Time in the scan room is typically 30-45 minutes. Due to
the highly penetrating nature of the 511 keV gamma rays
requiring significant lead thicknesses for attenuation,
locating the uptake rooms and scan room away from areas
fully occupied by members of the general public is the
most cost effective method of protection.
AAPM Position Statement (2/2004)
Scan Room Design: The Ct scanner and the patient should
both be considered as sources of radiation to personnel
and the public. The CT scan shielding design should be
done as usual, noting however, that the workload for a
PET/CT will be lower than for a traditional CT.
When considering the patient as a source, the following
should be accounted for:
patient self-shielding of the activity
radionuclide decay since injection time
loss of activity due to voiding
broad beam attenuation factors should be utilized
gantry attenuation
AAPM Position Statement (2/2004)
For shielding design guidelines, the following annual design
goals are recommended:
General Public: 1 mSv/year
Occupationally exposed workers (as described by NCRP
Report # 134): 5 mSv/year
In meeting these design goals, occupancy factors for low
occupancy areas such as toilets or outdoor areas as
described in current NCRP reports may be utilized.
AAPM Position Statement (2/2004)
Following installation, a Qualified Medical Physicist should
determine the adequacy of the shielding.
A Qualified Medical Physicist should specify and review the
Quality Assurance Program for the PET and PET/CT
The Executive Committee, Science Council and Radiation
Protection Committee of the AAPM have approved this
statement.
Melissa C. Martin, M.S., FACR
Therapy Physics, Inc.
9156 Rose St., Bellflower, CA 90706
562-804-0611 FAX: 562-804-0610
Cell Phone: 310-612-8127
e-mail: MelissaMartin@Compuserve.com